Shortages of fossil fuel materials in the recent past have spurred much speculation regarding the feasibility of economies based on other energy sources. One such scenario is a hydrogen-fueled economy. Hydrogen has the highest energy density per unit weight of any chemical. Many projections have been made for an economy based on this element, but the technology is not yet in place to effect such a dramatic change in the world economy. Hydrogen is, however, a technically attractive source of fuel and energy storage. It is essentially non-polluting, the major by-product of combustion being H.sub.2 O, and can be made from readily available and abundant raw materials.
While it is well known that hydrogen can be stored as a compressed gas or cryogenically as a liquid, other less energy-intensive and more convenient means are required for widespread utilization of hydrogen as a source of stored energy. It is known that some metals and metal alloys are capable of storing hydrogen reversibly within their lattice. This characteristic may be exploited by exposing the metal or metal alloy to a large pressure of hydrogen, impregnating the metal or metal alloy with hydrogen and later recovering the stored hydrogen by subjecting the impregnated metal or alloy to a change in temperature or pressure.
Recently, amorphous metal alloy materials have been reported as having the ability to store hydrogen reversibly. Amorphous metal alloy materials have become of interest due to their unique combinations of mechanical, chemical and electrical properties. Amorphous metal materials have compositionally variable properties including high hardness and strength, flexibility, soft magnetic and ferroelectronic properties, very high resistance to corrosion and wear, unusual alloy compositions, and high resistance to radiation damage. The unique combinations of properties possessed by amorphous metal alloy materials may be attributed to the disordered atomic structure of amorphous materials that insures that the material is chemically homogeneous and free from the extended defects that are known to limit the performance of crystalline materials.
Novel amorphous metal compositions for reversible hydrogen storage are disclosed in U.S. Ser. No. 717,428, which disclosure is incorporated herein by reference. This disclosure teaches compositionally graded reversible hydrogen storage materials comprising amorphous metal alloys of the formula: EQU A.sub.a M.sub.b M'.sub.c
wherein
A is at least one metal selected from the group consisting of Ag, Au, Hg, Pd and Pt; PA1 M is at least one metal selected from the group consisting of Pb, Ru, Cu, Cr, Mo, Si, W, Ni, Al, Sn, Co, Fe, Zn, Cd, Ga and Mn; and PA1 M' is at least one metal selected from the group consisting of Ca, Mg, Ti, Y, Zr, Hf, Nb, V, Ta and the rare earths; and PA1 a ranges from greater than zero to about 0.80; PA1 b ranges from zero to about 0.70; and PA1 c ranges from about 0.08 to about 0.95. PA1 (a) combining a bulk hydrogen storage material with an A-containing material to obtain a mixture thereof; PA1 (b) sealing the mixture in a mechanical milling device under an inert atmosphere; and PA1 (c) milling the mixture.
wherein
These amorphous compositions are not affected by phase separation or hydrogen embrittlement. Further, the above amorphous compositions have the ability to store hydrogen without exhibiting any significant signs of surface passivation or corrosion after repeated charge/discharge cycles.
Known processes suitable for the production of amorphous alloy materials include solid state reaction methods. U.S. Pat. No. 4,564,396 discloses a process for the formation of metastable solid, amorphous materials by solid state reaction, by diffusion of a metallic component into a solid compound, or by diffusion of a gas into an intermetallic compound.
Another method of procuding metastable amorphous or fine crystalline alloy materials is disclosed in U.S. Pat. No. 4,640,816, which recites the production of bulk alloy materials by reacting cold-worked, mechanically deformed filamentary precursors, such as metal powder mixtures or intercalated metal foils.
Similar mechanical alloying techniques are disclosed elsewhere in the literature primarily for the production of homogeneous mixtures or dispersions of metal-nonmetal components, such as in U.S. Pat. No. 3,737,300 disclosing wrought composite titaniferous and/or zirconiferous metal powders and U.S. Pat. No. 4,557,893 disclosing a process for producing composite materials by mechanical alloying.
However, while numerous methods and techniques are known for producing amorphous alloy materials, the field lacks a method by which compositionally graded alloys can be efficiently synthesized resulting in a substantial amount of one component being concentrated on the surface of a bulk material, while maintaining the desired characteristics of the resulting material. This area of technology also lacks a process which allows for control of processing parameters such that a compositionally graded structure can be achieved.
It is therefore, an object of this invention to provide a process by which compositionally graded amorphous metal alloys can be produced.
It is a further objection of this invention to provide a surface alloying process for the production of compositionally graded amorphous metal alloys having a substantial amount of one component on the surface of the bulk alloy component.